Transportin-2 plays a critical role in nucleocytoplasmic shuttling of oestrogen receptor-α

Oestrogen receptor-α (ERα) shuttles continuously between the nucleus and the cytoplasm, and functions as an oestrogen-dependent transcription factor in the nucleus and as an active mediator of signalling pathways, such as phosphatidylinositol 3-kinase (PI3K)/AKT, in the cytoplasm. However, little is known regarding the mechanism of ERα nucleocytoplasmic shuttling. In this study, we found that ERα is transported into the nucleus by importin-α/β1. Furthermore, we found that Transportin-2 (TNPO2) is involved in 17β-oestradiol (E2)-dependent cytoplasmic localisation of ERα. Interestingly, it was found that TNPO2 does not mediate nuclear export, but rather is involved in the cytoplasmic retention of ERα via the proline/tyrosine (PY) motifs. Moreover, we found that TNPO2 competitively binds to the basic nuclear localisation signal (NLS) of ERα with importin-α to inhibit importin-α/β-dependent ERα nuclear import. Finally, we confirmed that TNPO2 knockdown enhances the nuclear localisation of wild-type ERα and reduces PI3K/AKT phosphorylation in the presence of E2. These results reveal that TNPO2 regulates nucleocytoplasmic shuttling and cytoplasmic retention of ERα, so that ERα has precise functions depending on the stimulation.